scope.$digest() not updating local scope - angularjs

Question

In an angular app running on 1.3.0-beta.15, I have a directive that looks like this:
.directive('digester', function() {
return {
scope: {
object: '=',
digestTrigger: '='
},
templateUrl: 'some-template.html,
link: function(scope) {
scope.digestTrigger = function() {
scope.$digest();
}
}
};
});
a template that looks like this:
<div>{{object.title}}</div>
a service somewhere that looks like this:
.service('serviceToCallDigest', function() {
return {
object: {
title: 'someTitle'
},
digestTrigger: function() {},
someFunction: function() {
this.object.title = 'newTitle';
this.digestTrigger();
}
};
});
and a controller that gets the service injected like this:
.controller('someCtrl', function($scope, serviceToCallDigest) {
$scope.serviceToCallDigest = serviceToCallDigest;
}
and wires up the directive like this:
<digester
digest-trigger="serviceToCallDigest.digestTrigger"
object="serviceToCallDigest.object">
</digester>
Whats happening here is, as the directive gets the digestTrigger injected (as an empty function) it sets the function to fire the directives local $digest() function, and by that enabeling code from outside the directive to trigger the local digest whenever appropriate.
I know that $apply() is adviced to use, which fires $digest() at the $rootscope level, but I'm in a perfomance critical situation, where I need to update directives locally.
This setup has worked fine until upgrading to 1.3.0-beta.16 or newer versions. Now the new object.title is not passed to the directive and updated in the view, when someFunction is called.
Does anyone know why this is the case and what to do to work around it?